A conventional transversal filter used as a waveform equalizer automatically controls tap coefficients of delay elements having an inter-tap delay identical to a signal period. The filter is basically stable in view of the fact that it is of a non-cyclic type.
In a magnetic recording and reproducing apparatus for recording and reproducing a digital information signal through the use of a partial response method, there is used a waveform equalizer which employs the transversal filter. In such a filter, a reproduced digital information signal is adaptively adjusted according to the difference between an output of the filter and an estimated value thereof in order to suppress the inter-symbol interferences in the reproduced digital information signal, wherein the estimated value is obtained by mapping the output to one of predetermined ternary values. The output of the filter is subjected to a symbol decoder, e.g., Viterbi decoder, and an error correction device, e.g., RS (Reed Solomon) decoder.
FIG. 1 depicts a reproducing circuit 100 of the aforementioned conventional magnetic recording and reproducing apparatus. Specifically, a reproduced information signal, which is obtained by scanning a magnetic tape T with the aid of a magnetic head H installed on a rotary drum (not shown), is amplified to a predetermined level by a pre-amplifier (PA) 10 and then noise components contained in the amplified information signal are removed by a filter 20. Thereafter, the filtered information signal is converted into a digital information signal by an analog-to-digital converter (ADC) 30. At a direct current (DC) controller 40, the digital information signal is adjusted to issue a DC level as a reproduced digital information signal to a waveform equalizer 50. As shown in FIG. 1, the waveform equalizer 50 includes four delay circuits 51-54, five multipliers 55-59, an adding circuit 60, a coefficient updating circuit 70, a level detector 80 and a subtractor 90.
The delay circuits 51-54 sequentially delay the reproduced digital information signal applied thereto by a predetermined amount by each of them. The reproduced digital information signal from the DC controller 40 and each of the delayed digital information signals are then supplied to respective corresponding multipliers 55-59, wherein each signal applied to each corresponding multiplier is multiplied by a corresponding tab coefficient outputted from the coefficient updating circuit 70 which will be described later, thereby obtaining outputs from the multipliers 55-59 weighted by the tab coefficients. Thereafter, the adding circuit 60 adds the outputs of the multipliers 55-59 together and provides the sum, as an equalized output signal, to a symbol decoder and an error correction device (not shown), the level detector 80 and the subtractor 90. The level detector 80 compares the equalized output signal with predetermined thresholds to detect a level of the equalized output signal and provides the detected level to the subtractor 90.
At the subtractor 90, an amplitude error of the equalized output signal with respect to an expectation value of the equalized output signal is derived by calculating the difference between the detected level from the level detector 80, i.e., the expectation value, and the equalized output signal of the adding circuit 60, wherein the expectation value is one of, e.g., ternary values of -1, 0, 1. Subsequently, the amplitude error is fed to the coefficient updating circuit 70 which outputs the tap coefficients by multiplying the amplitude error by the reproduced digital information signal and the outputs of the delay circuits 51-54. By multiplying the reproduced digital information and the delayed signals thereof by the tap coefficients, the waveform equalization of the reproduced digital information signal can be accomplished by suppressing the inter-symbol interferences contained therein.
However, in the conventional waveform equalizer, there may occur a divergence of the tap coefficients in the event that the reproduced digital information signal which is severely distorted due to a damage to the tape is supplied to the equalizer. Once the tap coefficients have been diverged, a further coefficient updating operation is not performed in the waveform equalizer although the supply of the normal reproduced digital information signal thereto is resumed. Therefore, it is desirable to take an action for coefficient updating of the tap coefficients to their optimum values immediately when the tap coefficients are diverged.